With the recent trend toward weight reduction and size reduction in electrical products, development of a lithium secondary battery having a high energy density has now been in progress. There is also a desire for improvements in various battery characteristics as a result of the spread of fields to which lithium secondary batteries are applied.
Moreover, in the case that a lithium secondary battery is used as a backup power source at the time of the power failure or a power source for portable devices, in order to compensate self-discharge, there is usually used a continuous charge method (trickle charge) in which a very weak current is always applied to maintain the battery in a charged state.
A secondary cell using metal lithium as the negative electrode suffers from a problem that metallic lithium grows up to form dendrite after repeated charging/discharging, and the dendrite reaches the positive electrode to thereby cause short-circuit failure inside the cell, which is a largest obstacle to the practical use of the lithium secondary cell using metal lithium as the negative electrode. On the contrary, in the case of a nonaqueous electrolyte secondary battery using as a negative active material a carbonaceous material such as coke, artificial graphite or natural graphite capable of intercalating or releasing lithium, lithium does not grow up to dendrite and hence battery life and safety can be improved. In particular, a nonaqueous electrolyte secondary battery using a graphite-based carbonaceous material such as artificial graphite or natural graphite attracts attention as a battery capable of satisfying the requirement of high capacity.
However, in the secondary battery having a negative electrode of a graphite-based carbonaceous material, the electrolyte may decompose on the surface of the electrode during charging and discharging and thus decreased efficiency of charging and discharging, decreased cycle characteristics, increased inner pressure of the battery caused by generated gas, and the like may be sometimes induced.
As a method for obtaining a battery of high capacity, for the purpose of increasing the amount of active material of the electrode, it is general to densify an electrode layer by pressurization in order to reduce the voids in the electrode layer formed on the current collector of the electrode as far as possible. However, when the voids in the battery are reduced, the inner pressure of the battery remarkably increases even when the amount of gas generation by the decomposition of the electrolyte is only a little.
Therefore, with regard to the lithium secondary battery, it is required to suppress the decomposition of the electrolyte on the electrode surface.
In order to suppress the electrolyte of the nonaqueous electrolyte secondary battery using a graphite-based negative electrode, it is proposed to use a nonaqueous solvent containing a cyclic carbonic ester having a carbon-carbon unsaturated bond in the molecule, such as vinylene carbonate or a derivative thereof (e.g., see Patent Document 1). When the nonaqueous solvent is used, a film formed by reductive decomposition of the cyclic carbonic ester having an unsaturated bond on the surface of the negative electrode can suppress excessive decomposition of the nonaqueous solvent to thereby improve cycle characteristics. However, from the experiments of the present inventors, it has been revealed that the secondary battery using a nonaqueous solvent containing a cyclic carbonic ester having a carbon-carbon unsaturated bond in the molecule has a problem that gas generation increases when continuous charging is conducted, although it exhibits excellent cycle characteristics. It seems that this is because gas generation does not decrease because activity of the positive electrode does not decrease in the continuous charging in which charging is continued at a constant voltage.
In such a situation, there have been reported numerous methods of adding various additives for improving initial capacity, rate characteristics, cycle characteristics, high-temperature storage characteristics, low-temperature characteristics, continuous charge characteristics, self-discharge characteristics, overcharge-preventing characteristics, and the like. For example, as methods for improving cycle characteristics, there are disclosed the addition of a divalent sulfonate compound such as 1,4-butanediol dimethanesulfonate or propylene glycol dimethanesulfonate (e.g., see Patent Documents 2 and 3), the incorporation of an alkanesulfonic alkyl ester (otherwise, alkyl alkanesulfonate) (e.g., see Patent Document 4), and the fact that a cycle capacity is increased by incorporating a silyl sulfate such as bis(trialkylsilyl)sulfate (e.g., see Patent Document 5).
Moreover, it is reported that the incorporation of 1,4-thioxane-1,1-dioxide in the electrolyte results in the formation of a complex of cobalt eluted from a positive electrode with 1,4-thioxane-1,1-dioxide to stabilize the cobalt ion and to suppress the precipitation of cobalt on the negative electrode and, as a result, the decomposition of the electrolyte is suppressed and high-temperature storage and high-temperature charge/discharge cycle characteristics are improved (e.g., see Patent Document 6). In addition, an electrolyte containing a compound having a molecular weight of less than 500 and having an NS structure in which nitrogen and sulfur are bonded is also reported (i.e., see Patent Document 7).
Furthermore, in order to improve battery characteristics, safety and the like, the incorporation of a fluorine-containing aromatic compound in the nonaqueous solvent is known (i.e., see Patent Documents 8 to 13). However, a method for suppressing gas generation at continuous charging is not described in any of the literatures.
As a method for improving battery characteristics at continuous charging, a secondary battery using an electrolyte containing a phosphoric ester is proposed (e.g., see Patent Document 14). However, based on the experiments of the present inventors, the battery is insufficient in battery characteristics after continuous charging.
[Patent Document 1]
JP-A-8-45545
[Patent Document 2]
JP-A-2000-133304
[Patent Document 3]
JP-A-2001-313071
[Patent Document 4]
JP-A-9-245834
[Patent Document 5]
JP-A-2001-176548
[Patent Document 6]
JP-A-2002-134170
[Patent Document 7]
JP-A-2002-280063
[Patent Document 8]
JP-A-10-112335
[Patent Document 9]
JP-A-11-329496
[Patent Document 10]
JP-A-2000-106209
[Patent Document 11]
JP-A-2001-185213
[Patent Document 12]
JP-A-2001-256996
[Patent Document 13]
JP-A-2002-83629
[Patent Document 14]
JP-A-11-233140
Recently, a higher performance for a lithium secondary battery has been increasingly required. That is, it is required to satisfy various characteristics such as high capacity, cycle characteristics, high-temperature storage characteristics, and continuous charge characteristics at a high level. In particular, since mobile products are frequently utilized out of doors and a demand for office notebook-size personal computers is increasing, the improvement of continuous charge characteristics is particularly much desired in recent years.
At the time when notebook-size personal computers are used in offices, AC adaptors are used as power sources in most cases and hence the secondary batteries in the personal computers are continuously charged. In such continuous charging, gas generates owing to the decomposition of the electrolyte. In the case of a cylindrical battery in which a safety valve is actuated with detecting inner pressure at an abnormal event such as overcharge, the safety valve may be actuated at continuous charging when a large amount of gas generates.
Moreover, in a prismatic battery without a safety valve, when the amount of gas is large, it becomes necessary to house bare cells in a larger case so that no change is observed in appearance, which leads to decrease in energy density of the whole battery pack. When the gas generation is much more, there is a risk of case burst.
Therefore, with regard to the continuous charge characteristics, not only high recovered capacity and small capacity degradation after test but also suppression of gas generation during the charging are strongly required. However, the electrolytes hitherto proposed have often provided no improvement of battery characteristics such as the continuous charge characteristics.
For example, the use of the electrolyte containing an alkanesulfonic alkyl ester disclosed in the above Patent Document 4 improves capacity deterioration but the improvement is only a small degree and no effect on the improvement of the continuous charge characteristics is exhibited.
Moreover, since the silyl sulfates disclosed in Patent Document 5 are highly corrosive and react with cell current collectors, storage characteristics, particularly at a high temperature is deteriorated when the compounds are incorporated in electrolytes. Furthermore, the toxicity of the silyl sulfates is unclear in many points but based on the inference from the fact that dimethyl sulfate, which is an analogous compound, is regulated by the Ordinance on Prevention of Hazards Due to Specified Chemical Substances owing to its strong corrosiveness and toxicity to central nerve system, it may induce a great risk in safety to incorporate them in electrolytes.
The electrolyte containing 1,4-thioxane-1,1-dioxide (Patent Document 6) improves the capacity deterioration during high-temperature storage but the improvement is only a small degree and no effect on the improvement of the continuous charge characteristics is exhibited.
Furthermore, 1,1′-sulfonyldiimidazole, N-bismethylthiomethylene-p-toluenesulfonamide, and 1-p-tolylsulfonylpyrrole disclosed in Patent Document 7 cannot improve both of the storage characteristics at a relatively high temperature of 80° C. or higher and the continuous charge characteristics.
Accordingly, it is also required to improve the continuous charge characteristics in addition to high capacity, high-temperature storage characteristics, load characteristics, and cycle characteristics. As the continuous charge characteristics, not only reduction of capacity deterioration but also suppression of gas generation is strongly requested.